1. Field of the Invention
The present invention relates to a polishing pad and a surface polishing method, which are suitable for a rotary surface polishing apparatus that polishes a surface of a workpiece such as a disk substrate of a magnetic storage medium for a fixed magnetic disk unit, a silicon wafer for a semiconductor, and liquid crystal glass and the like by a chemo-mehanical-polishing (CMP) method.
2. Description of Related Art
To obtain a flat surface of the disk substrate of the magnetic storage medium, the surface of a disc-shaped substrate made of an aluminum alloy is plated with Nixe2x80x94P of about 10 xcexcm, and then both sides of the disk substrate are polished (lapped) before the formation of a magnetic layer. Likewise, the flat surfaces of the silicon wafer and the liquid crystal glass and the like are obtained by polishing both sides of their substrates.
On the other hand, a rotary surface polishing machine for polishing the disk substrate, the silicon wafer and the like is widely used which comprises a pair of upper and lower platens, polishing pads attached to the inner surfaces of the platens, and a carrier that is mounted between the upper and lower platens to hold the workpiece. The workpieces, which are inserted into set holes formed in the carrier, are pinched by the polishing pads attached to the platens from above and below. In this state, a slurry is dropped between the polishing pads and the workpiece from the direction of the upper platen while the upper and lower sides of the workpiece are simultaneously polished by rotating the upper and lower platens and the carrier.
Next, there will be described the structure of the rotary surface polishing machine (lapping machine) and the polishing operation with reference to FIGS. 4 through 6. In FIGS. 4(a), 4(b) and FIG. 5, reference numeral 1 denotes an upper platen; 2, a lower platen; 3, a carrier mounted between the upper platen 1 and the lower platen 2; 4, a workpiece (e.g., the disk substrate and the silicon wafer and the like) to be polished; and 5, polishing pads attached to the inner surfaces of the upper and lower platens 1 and 2.
As shown in the drawing, the carrier 3 is constructed as a planetary gear that rotates and revolves a disc 3a through a gear mechanism 3b. A plurality of set holes are formed in the disc 3a (four set holes are formed in FIG. 4(b)), and the workpieces 4 are inserted into the set holes one by one.
With this arrangement, the workpieces 4, which are inserted one by one into the set holes formed in the disc 3a of the carrier 3, are pinched by the polishing pads 5 attached to the inner surfaces of the platens 1 and 2 from above and below. In this state, a slurry 6 is dropped through a slurry supply hole 1a formed in the upper platen 1 while the upper and lower platens 2 are rotated in reverse directions with the rotation and revolution of each carrier. Therefore, the workpiece 4 as well as the carrier 3 moves on a plane between the upper and lower platens 1 and 2, and the upper and lower sides of the workpiece 4 are polished by the polishing pads 5 and the slurry 6. In the prior art, the slurry 6 is ordinarily produced by finely crushing a hard solid matter composed mainly of metal oxide and carbon by a mill or the like, and dispersing the classified fine powder with a predetermined grain size as abrasive grains in a chemical with an etching function.
On the other hand, the polishing pads 5 are now ordinarily made of soft plastic foam. As shown in a conceptual drawing of FIG. 6, the polishing pads 5 are conventionally structured in such a manner that a nap layer 5b made of plastic foam is deposited on a sheet-shaped base layer 5a. A honeycomb pore structure is constructed in the nap layer 5b in the following manner. Polyethylene, polyurethane resin, and the like are foamed and spread in the shape of a sheet, and skin layers (non-foaming layers which form the surface of the plastic foam) which form the surface of the nap layer 5b are buffed to horizontally cut pores (foam) 5b-1 in the layer. This forms pore cavities in the surface of the nap layer 5b. 
In the polishing pads 5 with the above-mentioned structure, the nap layer 5b rubbing the workpiece 4 has an uneven surface having the honeycomb pore structure. Crater-shaped cavities of the pores 5b-1 hold the slurry 6 dropped from the outside during the polishing. As shown in FIG. 6, when the workpiece 4 moves relatively to the upper and lower polishing pads 5, the slurry held in the pores is squeezed out to polish the surface of the workpiece 4. The sludge deposited with the progress of the polishing and other mixed alien matters are captured into and held in the cavities of the pores 5b-1 in order to prevent the surface of the workpiece from being scratched and the like.
The above-mentioned method, in which the workpiece is polished by a combination of the polishing pads and the slurry with the conventional structure, has the following problems to be solved:
1) As shown in the conceptual drawing of FIG. 6, the surface of the nap layer 5b made of the plastic foam in the conventional polishing pad 5 is uneven, and only cut parts of a wall surrounding the pores 5b-1 locally contact with and slide on the workpiece 4 during the polishing. Thus, the polishing pads 5 cannot uniformly contact with the entire surface of the workpiece 4. Therefore, the workpiece 4 cannot be polished uniformly, and this results in a fine xe2x80x9cwavinessxe2x80x9d on the polished surface of the workpiece 4. It is therefore difficult to ensure a surface quality required by a product specification. The xe2x80x9cwavinessxe2x80x9d as well as xe2x80x9csurface roughnessxe2x80x9d is an item to be measured for evaluating the surface quality with respect to the disk substrate, the silicon wafer and the like. The xe2x80x9cwavinessxe2x80x9d is represented by a waving amount (Wa) of a surface image per unit area observed by an optical non-contact surface roughness gauge (ZYGO) in an angstrom (xc3x85). Particularly, if the xe2x80x9cwavinessxe2x80x9d is increased on the disk substrate for use in the fixed magnetic disk unit used in combination with a floating magnetic head, a floating characteristic of the magnetic head is deteriorated. It is therefore important to reduce the xe2x80x9cwavinessxe2x80x9d as much as possible during the polishing.
2) The abrasive grains of micro powder (whose particles have square surfaces) obtained by crushing and classifying a solid matter as mentioned previously are ordinarily mixed in the conventional slurry 6. The abrasive grains, the sludge and the like easily sediment and agglutinate in the slurry of this kind. If this slurry is used in combination with the conventional polishing pads 5 described with reference to FIG. 6, the alien matters such as the abrasive grains and the sludge in the slurry are caked in the pores 5b-1 formed in the surface of the nap layer 5b during the polishing although a large amount of slurry can be held on the polishing pads. If the caked alien matters are left as they are, they may rub the surface of the workpiece during the polishing to thereby form scratches, which may cause troubles. Therefore, in the prior art, a brush, a jet stream or the like frequently cleans the surfaces of the polishing pads 5 in a short cycle in order to remove the alien matters (caked matters) adhered to the polishing pads. This maintaining (cleaning) operation, however, requires a lot of time and effort, and it is necessary to stop running the polishing machine during the maintaining operation. This affects the operating rate of the polishing machine.
3) The conventional polishing pad has much unevenness in the nap layer thereof after the manufacture. In order to polish a product mounted on the surface polishing machine, a running-in is performed in advance to fair the surface of the polishing pad by polishing a dummy workpiece. The running-in requires a lot of time, which is one of the causes of the decrease in the operating rate of the polishing machine.
4) Recently, a colloidal slurry has been mainly used as the slurry in order to improve the polishing accuracy, prevent the abrasive grains from caking, and improve the maintainability of the polishing pads. The use of the colloidal slurry in combination with the conventional polishing pads, however, results in the polishing unevenness and the fine xe2x80x9cwavinessxe2x80x9d, and it is difficult to make full use of the characteristics owned by the colloidal slurry.
It is therefore an object of the present invention to provide a polishing pad and a surface polishing method, which are improved so as to increase the workpiece polishing accuracy and uniformly polish the surface of the workpiece particularly in combination with a colloidal slurry.
The above object can be accomplished by providing a polishing pad which comprises a base layer and a sheet-shaped nap layer laminated on the base layer and made of soft plastic foam, the polishing pad wherein: the nap layer is formed of closed pores, whose surfaces are covered with skin layers and whose pores are involved and closed within the nap layer.
In a preferred mode of the present invention, a flat surface of the nap layer is obtained by buffing external surfaces of the skin layers in the closed pores forming the nap layer to such an extent as not to open the pores involved in the nap layer.
In another preferred mode of the present invention, the base layer is made of high hardness resin.
In yet another preferred mode of the present invention, the base layer is a non-woven fabric made of synthetic fiber.
In yet another preferred mode of the present invention, the base layer is a woven fabric made of synthetic fiber.
The polishing pad of the present invention is more advantageous in view of functions and maintenance compared with the conventional polishing pad in which pores (air bubbles) of a plastic foam are opened in the surface. The advantages are as follows.
a) A skin layer of a plastic foam forming the nap layer of the polishing pad directly contacts with the whole surface of the polished surface of the workpiece to polish the workpiece, and the foaming pores involved in the layer serve as a cushion. Thus, the surface of the workpiece can be polished with a substantially constant polished surface pressure, and this prevents the polishing unevenness and the xe2x80x9cwavinessxe2x80x9d on the surface of the workpiece, which are the problems of the polishing with the conventional polishing pads. Therefore, the workpiece can be polished with an excellent surface quality. Moreover, the slurry supplied to the surface of the polishing pad spreads over the whole surface of the workpiece to polish the workpiece, and this achieves a high polishing performance.
b) The slurry dropped from the outside during the polishing flows to the outside of a system after flowing between the workpiece and the surfaces of the skin layers in the polishing pads. Thus, alien matters such as sludge can be quickly discharged to the outside of the system in company with the slurry without being adhered to or remaining on the polishing pads. This prevents the formation of defects such as scratches in the surface of the workpiece, which results from the caking of the sludge adhered to the polishing pads. Moreover, the frequency of cleaning the polishing pads can be decreased, so that the polishing pads can be used continuously for a long period of time without maintenance.
c) The whole surfaces of the polishing pads are covered with the flat skin layers of the closed pores, and thus, the running-in performed initially can be shortened. This enables a quick setup of the polishing machine, and improves the operating rate of the polishing machine.
On the other hand, in a surface polishing method of the present invention, the polishing pad that is constructed in the above-mentioned manner is attached to platens of the surface polishing machine, and the workpiece is polished by using a colloidal slurry. The colloidal slurry is obtained by dispersing micro powder of colloidal silica as abrasive grains in a dispersion medium.
The colloidal silica is produced by a chemical process, and is different from those produced by mechanically crushing a solid matter by a mill or the like. The colloidal silica is a hard micro powder, whose grain size is 0.02-0.1 xcexcm and which has a flat surface. The colloidal slurry, in which the colloidal silica as abrasive grains are dispersed in the dispersion medium, has a high dispersiveness, and thus, the abrasive grains are not easily caked. There is little possibility that the abrasive grains are caked to cause defects such as scratches in the surface of the workpiece during the polishing. Thus, the use of the colloidal slurry in combination with the polishing pads of the present invention achieves a high polishing performance and an excellent surface quality since the characteristics of the colloidal slurry are made full use of.